This invention relates to radio communication systems and more particularly to measurement of received signal power in such systems.
In the continuing evolution of mobile cellular radio standards like GSM and wideband code division multiple access (WCDMA), new transmission techniques like orthogonal frequency division multiplex (OFDM) will be used in new cellular communication systems. Furthermore, to migrate smoothly from existing cellular systems to new high-capacity, high-data-rate systems in the existing radio spectrum, the new systems have to be able to operate with flexible communication channel bandwidths.
One such new flexible cellular communication system is called Third Generation Long Term Evolution (3G LTE), which is currently being standardized by the Third Generation Partnership Project (3GPP). The 3G LTE specifications can be seen as an evolution of the current WCDMA specifications also promulgated by the 3GPP. A 3G LTE system will use OFDM as a multiple access technique (called OFDMA) in the downlink (DL) from system nodes to user equipments (UEs), will operate with channel bandwidths ranging from about 1.4 megahertz (MHz) to about 20 MHz, and will support data rates up to 100 megabits per second (Mb/s) on the largest-bandwidth channels. Besides high-data-rate services, 3G LTE systems are expected to provide low-data-rate services, such as speech. Because 3G LTE is designed for packet data according to the familiar transmission control protocol/internet protocol (TCP/IP), it is expected that the service that carries speech will use voice-over-IP (VoIP).
In an OFDMA communication system, the data stream to be transmitted is portioned among a number of narrowband subcarriers that are transmitted in parallel. In general, a resource block devoted to a particular UE is a particular number of particular subcarriers used for a particular period of time. Different groups of subcarriers can be used at different times for different users. Because each subcarrier is narrowband, each carrier experiences mainly flat fading, which makes it easier for a UE to demodulate each subcarrier. OFDMA communication systems are described in the literature, for example, U.S. Patent Application Publication No. US 2008/0031368 A1 by B. Lindoff et al.
FIG. 1 depicts a typical cellular communication system 10. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. In general, each RNC directs calls to and from a UE, such as a mobile station (MS), mobile phone, or other remote terminal, via appropriate base station(s) (BSs), which communicate with each other through DL (or forward) and uplink (UL, or reverse) channels. In FIG. 1, RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26.
Each BS, or Node B in 3G vocabulary, serves a geographical area that is divided into one or more cell(s). In FIG. 1, BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26, although a sector or other area served by signals from a BS can also be called a cell. In addition, a BS may use more than one antenna to transmit signals to a UE. The BSs are typically coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. The RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
It should be understood that the arrangement of functionalities depicted in FIG. 1 can be modified in 3G LTE and other communication systems. For example, the functionality of the RNCs 12, 14 can be moved to the Node Bs 22, 24, 26, and other functionalities can be moved to other nodes in the network. It will also be understood that a base station can use multiple transmit antennas to transmit information into a cell/sector/area, and those different transmit antennas can send respective, different pilot signals.
FIG. 2 is a frequency-vs.-time plot showing an arrangement of DL subcarriers in an OFDM communication system, such as a 3G LTE system. As shown in FIG. 2, a resource block includes twelve subcarriers spaced apart by fifteen kilohertz (kHz), which together occupy 180 kHz in frequency and 0.5 millisecond (ms) in time, or one time slot. FIG. 2 shows each time slot including seven OFDM symbols, or resource elements (REs), each of which has a short (normal) cyclic prefix, although six OFDM symbols having long (extended) cyclic prefixes can also be used in a time slot. It will be understood that resource blocks can include various numbers of subcarriers for various periods of time.
An important aspect of a 3G LTE system is the mobility of the UEs, and so fast and efficient cell search and received signal power measurements are important for a UE to get and stay connected to a suitable cell, which can be called the “serving cell”, and to be handed over from one serving cell to another. Furthermore, operators will deploy LTE gradually in time and location, and so Inter-Radio Access Technology (IRAT) mobility will be an important functionality. Mobility from a GSM/WCDMA system to an LTE system is just one of many examples of IRAT mobility.
In current 3G LTE specifications, handover decisions are based on measurements of reference signal received power (RSRP), which can be defined as the average received signal power of reference signals or symbols (RS) transmitted by a Node B. A UE measures RSRP on its serving cell as well as on neighboring cells that the UE has detected as a result of a specified cell search procedure.
The RS, or pilots, are transmitted from each Node B at known frequencies and time instants, and are used by UEs for synchronization and other purposes besides handover. Such reference signals and symbols are described for example in Sections 6.10 and 6.11 of 3GPP Technical Specification (TS) 36.211 V8.4.0, Physical Channels and Modulation (Release 8), September 2008.
RS are transmitted from each of possibly 1, 2, or 4 transmit antennas of a Node B on particular REs that can be conveniently represented on the frequency-vs.-time plane as depicted in FIG. 3. It will be understood that the arrangement of FIG. 3 is just an example and that other arrangements can be used.
FIG. 3 shows two successive time slots, indicated by the vertical solid lines, which can be called a sub-frame. FIG. 3 also shows two resource blocks, which are indicated by the dashed lines. The frequency range depicted in FIG. 3 includes about twenty-six subcarriers, only nine of which are explicitly indicated. RS transmitted by a first transmit (TX) antenna of a Node B are denoted R and by a possible second TX antenna in the node are denoted by S. In FIG. 3, RS are depicted as transmitted on every sixth subcarrier in OFDM symbol 0 and OFDM symbol 3 or 4 (depending on whether the symbols have long or short cyclic prefixes) in every slot. Also in FIG. 3, the RSs in symbols 3 or 4 are offset by 3 subcarriers relative to the RS in OFDM symbol 0, the first OFDM symbol in a slot.
The artisan will understand that it is desirable for a UE to base its RSRP measurements in optimal ways on RS transmitted in the serving or other cell. A cell detected in a cell search procedure but not currently connected to the UE can be called “a detected neighboring cell”. Low signal-to-interference ratio (SIR) is a common situation for a detected neighboring cell because such a cell's signal power level at the UE is usually lower than the received power level of the serving cell. Different SIRs can call for different RSRP measurement methods.
Furthermore, a UE typically assumes that the characteristics of the DL channel are constant over a number of subcarriers (i.e., the channel is constant with frequency) and over a number of OFDM symbols (i.e., the channel is constant in time). Based on that assumption, the UE estimates the RSRP by coherently averaging received symbols over such a “constant” group to get a channel estimate Hi for a subcarrier i, computes the square of the absolute value of the channel estimate |Hi|2 to obtain a received signal power estimate over the “constant” group of symbols, and then computes a non-coherent average of such signal power estimates over several groups, e.g., an entire channel bandwidth, to determine an RSRP measurement (estimate). Two such assumed “constant” groups are indicated in FIG. 3 by the dashed lines.
In the arrangement depicted in FIG. 3, such a “simple” cell measurement method of coherent averaging followed by non-coherent averaging to estimate the RSRP can proceed as follows. The UE's baseband signal Yi corresponding to an RS Ri from TX antenna 1 can be written as follows:Yi1=Hi1Ri+Ei  Eq. 1and the UE's baseband signal corresponding to an RS Si from a possible TX antenna 2 can be written similarly as follows:Yi2=Hi2Si+Ei  Eq. 2from which the impulse response Hi of the channel can be estimated using the known RS symbols Ri, Si. It will be noted that the superscript 2 in Eq. 2 does not denote a square but a second TX antenna.
Coherent averaging of a number M of received reference symbols followed by non-coherent averaging of a number N of coherent averages (i.e., non-coherent averaging over N resource blocks) can be written as follows:
                              S          est                =                              1            N                    ⁢                                    ∑                              n                =                1                            N                        ⁢                                                                                                1                    M                                    ⁢                                                            ∑                                              m                        =                        1                                            M                                        ⁢                                          R                      ⁢                                                                                          ⁢                                              S                        m                        est                                                                                                                        n              2                                                          Eq        .                                  ⁢        3            in which Sest is the RSRP measurement (estimate) and RSest are channel response estimates based on the RS symbols Ri or Si. The non-coherent averaging is typically done over an entire UE measurement bandwidth (e.g., 1.4 MHz, or six pairs of resource blocks) to determine the total RSRP estimate.
Besides having a variable SIR, the DL channel commonly suffers from delay spread and Doppler shift, and so the channel is not constant as typically assumed, leading to increased probability of erroneous RSRP measurement values. A known solution to this problem of varying DL channels is to use more advanced methods of estimating the channel and signal power (e.g., methods based on Wiener filtering). Such more advanced methods are computationally intensive, consuming time, power, and/or hardware resources that are limited in many UEs, increase the complexity of the signal-power-estimate processing in a UE, and need to be done on each detected neighboring cell, all of which render this solution undesirable.
Inter-frequency cell measurements are typically done in inter-frequency measurement gaps, which is to say that the UE is configured to halt its reception of information from its serving cell in order to switch its received carrier frequency and do measurements on inter-frequency or IRAT cells. Depending, for example, on the relative timing of the inter-frequency gaps and the cells, a UE can either use the above-described “simple” method to do received signal measurements or it can not.
FIG. 4 depicts an example of inter-frequency cell measurements for the case of GSM cells (indicated by (A) and (B)) and an LTE cell that uses time-division duplex (TDD). Time is arranged along the horizontal direction in FIG. 4, and the succession of resource blocks of the LTE TDD cell is indicated in the middle of FIG. 4. The succession of GSM frames is indicated for two offset timings, with the radio set-up periods at the beginnings and ends of frames being indicated by the hatched areas. For GSM cells, the inter-frequency measurement gap available for IRAT cell measurements is only around 5.2 ms, which is the duration of the idle frame that occurs every twenty-six frames. For a LTE TDD cell, only a subset of the sub-frames can be used for RSRP measurements. The above-described “simple” method can thus be used only for some relative timings of the inter-frequency cells. The relative timing of GSM cell (A) shown in FIG. 4 is such that it allows the above-described “simple” measurement method to be used because all of the LTE resource blocks occur in the idle frame, during which the UE can re-tune its receiver to do cell measurements on the LTE cell. In contrast, the UE cannot use the above-described “simple” measurement method when a relative timing such as cell (B) exists.
Previous solutions to this problem always require more complicated channel and signal power estimate methods (for instance, methods based on Wiener filtering) in order to cope with such “worst case” scenarios in which the above-described “simple” measurement method cannot be used. As mentioned above, such more complicated methods are undesirable.
Therefore, there is a need for improved methods and apparatus for inter-frequency and IRAT cell measurements without significantly increasing the complexity of the measurements.